A multi-carrier communication system, such as a Discrete Multiple-Tone (DMT) system in the various types of Digital Subscriber Line (e.g. ADSL and VDSL) systems, carries information from a transmitter to a receiver over a number of tones. Each tone may be a group of one or more frequencies defined by a center frequency and a set bandwidth. The tones are also commonly referred to as sub-carriers or sub-channels. Each tone acts as a separate communication channel to carry information between a local transmitter-receiver device and a remote transmitter-receiver device.
DMT communication systems use a modulation method in which the available bandwidth of a communication channel, such as twisted-pair copper media, is divided into these numerous tones. A communication channel may also be known as a communication channel. The term communication channel is understood to refer generally to a physical transmission medium, including copper, optical fiber, and so forth, as well as other transmission mediums, including radio frequency (RF) and other physical or non-physical communication signal paths.
There are various types of interference and noise sources in a multi-carrier communication system. Interference and noise may corrupt the data-bearing signal on a tone as the signal travels through the communication channel and is decoded at the receiver. The transmitted data-bearing signal may further be decoded erroneously by the receiver because of this signal corruption.
A measure of the quality of signal carried by a tone is its Signal to Noise Ratio (SNR). SNR is the ratio of the received signal strength (power) over the noise strength in the frequency range of operation. High SNR results in high signal quality being carried by a tone. Another measure of signal quality is bit error rate (BER) for a given tone. BER is inversely related to SNR. Thus, when the SNR of a tone is low, BER of the tone is high.
The number of data bits or the amount of information that a tone carries may vary from tone to tone and depends on the relative power of the data-bearing signal compared to the power of the corrupting signal on that particular tone. In order to account for potential interference on the transmission line and to guarantee a reliable communication between the transmitter and receiver, each tone is designed to carry a limited number of data bits per unit time based on the tone's SNR using a bit-loading algorithm. The number of bits that a specific tone may carry decreases as the relative strength of the corrupting signal increases, that is when the SNR is low or the BER is high. Thus, the SNR of a tone may be used to determine how much data should be transmitted by the tone.
It is often assumed that the corrupting signal is an additive random source with Gaussian distribution and white spectrum. However, this assumption may not be true in many practical cases. Inter-symbol interference (ISI), uncancelled residual echo, radio-frequency interference (RFI), windowed background noise and phase error are some such noise sources that may not have a white, Gaussian distribution. Bit-loading algorithms, which are methods to determine the number of data bits per tone, are usually designed based on the assumption of additive, white, Gaussian noise. With such algorithms, the effects of noise sources that do not have a white, Gaussian distribution maybe overestimated (or underestimated), resulting in the Bit-loading algorithm allocating lower (or higher) data bits per tone than the tone can actually carry.